1. Field of the Invention
The invention relates to a calorimeter suitable for measuring the latent energy of a sample at a pressure of at least 1000 bars, especially the latent heat of change of state.
2. Description of the Prior Art
Conventional calorimetric measurements at pressures close to atmospheric pressure are carried out by placing a sample in a calorimetric chamber inside an adiabatic (i.e., thermally insulated) enclosure so that there is substantially no heat exchange with the outside environment. As the heat capacity of the chamber is known, the energy changes inside it on a change of state can be measured in terms of the variation in temperature of the chamber. The sensitivity of the measurement is set by the resolution of the temperature measuring system used to detect the temperature changes, varies in inverse ratio to the heat capacity of the chamber, and is limited by heat exchanges with the outside environment. It is possible to take these heat exchanges into account, however, by plotting the temperature against time.
When the measurements must be made at high pressures, the calorimeter chamber becomes a test chamber with walls which are sufficiently thick to withstand the pressures brought into play. The heat capacity of the chamber increases with its mass. It is estimated that in order to withstand pressures as high as 1,000 to 10,000 bars the test chamber should have a mass such that the sensitivity is reduced by at least two orders of magnitude, as compared with a conventional calorimeter, except, of course, at very low temperatures close to absolute zero, where calorimetry raises very special problems. Also, measurements made by the conventional method have not gone beyond the high pressure region (i.e., pressures less than or close to 1,000 bars). Certain very special forms of apparatus have been able to go as high as 2,000 bars. Bomb calorimeters used for measuring combustion energy release very large amounts of specific energy, which does not call for high sensitivity. In the very high pressure field under consideration, it is sometimes possible to determine latent energy by indirect methods which rely on measuring parameters which have a known relationship to the internal energy of the sample.
The number of measurable parameters of a sample in a test chamber at very high pressure is very small. Thus it was considered desirable to be able to carry out calorimetric measurements on samples in very high pressure conditions so as to determine in an indirect manner physical parameters which cannot be measured directly. Among these parameters, mention may be made of the coefficient of expansion in the very high pressure field.
Studying the desirable performance of a calorimeter usable in the range of very high pressures from 1,000 to 10,000 bars for studying phenomena involving mass energy variations of the order of 10.sup.-.sup.2 joules/gm led to the realization that the means for measuring the temperature variations would have to have a resolution of at least 10.sup..sup.-5 .degree. C.